1. Field of the Invention
The present invention relates to a method for the manufacture of a high-density, high-speed semiconductor device through using a refractory metal in a lift-off step.
2. Description of the Prior Art
With the recent progress of semiconductor integrated circuit technology toward large-capacity, high-density LSIs, miniaturization, surface planarization and multilevel metallization of respective structures forming the LSI, such as a field isolation structure, a gate electrode structure and an interconnection structure, are increasing in importance. One method that is effective for miniaturization, planarization and multilevel metallization is a lift-off process. In the prior art, a polymer such as resist and polyimide resin, aluminum and zinc oxide are employed in the lift-off process.
A conventional lift-off process using the resist is carried out in the following manner. In FIG. 1A, reference numeral 1 indicates a silicon substrate, and 2 designates a thermal oxide film or CVD SiO.sub.2 film formed on the silicon substrate 1. A pattern 3 reverse from a wiring pattern is formed on the film 2 using photoresist. An aluminum film 4 is deposited by evaporation to cover the surfaces of the film 2 and the photoresist pattern 3 as shown in FIG. 1B. Then, the resist pattern 3 is dissolved by ultrasonic washing in acetone to lift off the overlying aluminum film 4, thus forming a wiring pattern as shown in FIG. 1C. This method is free from side etching, and hence is advantageous in that a miniature pattern can be formed. The basic concept of the lift-off process is as shown in FIGS. 1A to 1C. Various methods have been proposed for facilitating the lift-off step.
A description will now be given, with reference to FIGS. 2A to 2D, of an example of forming a miniature and planar wiring layer by utilizing the conventional lift-off method. In FIG. 2A, reference numeral 1 indicates a silicon substrate; 2 disignates an SiO.sub.2 film; 13 identifies an aluminum film; and 14 denotes a resist pattern. The aluminum film 13 is selectively etched away using the resist pattern 14 as a mask to obtain a structure shown in FIG. 2B, on which an SiO.sub.2 or Si.sub.3 N.sub.4 film is deposited by sputtering to obtain a structure shown in FIG. 2C. Reference numeral 15 represents the deposited SiO.sub.2 or Si.sub.3 N.sub.4 film. The resist pattern 14 is dissolved by ultrasonic washing in acetone to lift off the overlying film 15, obtaining a planar structure depicted in FIG. 2D.
Another example using the prior art lift-off process will now be described with reference to FIGS. 3A to 3F. In FIG. 3A, reference numeral 1 indicates a silicon substrate; 2 designates a thermal oxide film; 33 identifies a polysilicon layer; and 34 denotes a resist pattern. The polysilicon film 33 is selectively etched away by a reactive ion etching method using the resist pattern 34 as a mask without causing side etching, thereby to obtain a structure shown in FIG. 3B. An insulating film, such as an SiO.sub.2 or Si.sub.3 N.sub.4 film, is formed on the surface of the structure by means of a thin film deposition method which is capable of depositing the film with a directionality in a direction perpendicular to the substrate surface, such as, for instance the ion beam sputter method, by which is obtained a structure shown in FIG. 3C. In FIG. 3C, reference numeral 35 represents an SiO.sub.2 film deposited on the resist pattern 34 or the thermal oxide film 2; and 38 signifies SiO.sub.2 films deposited on the side wall 37 of the resist pattern 34 and the side wall 36 of the polysilicon film 33. The directionality of the film deposition is excellent because the ions have directionality, but the SiO.sub.2 film 38 is deposited on the side walls to a thickness of a fraction of the thickness of the SiO.sub.2 film deposited on the flat surface portion. Accordingly, in the case where the SiO.sub.2 film 38 is not removed by etching, if it is thin and has a pinhole, the lift-off of the resist pattern 34 can be performed but the yield rate of the lift-off operation is low and a burr remains unleft after the lift-off. In this case, however, bonding of the SiO.sub.2 film 38 deposited on the side wall is loose and its etching rate is higher than that of the SiO.sub.2 film 35 deposited on the flat surface portion, so that the structure after etching away the SiO.sub.2 film 38 is such as shown in FIG. 3D. The resist pattern 34 is dissolved by ultrasonic washing with a solvent to lift off the overlying SiO.sub.2 film, providing a structure depicted in FIG. 3E. In this case, the top surfaces of the SiO.sub.2 film 35 and the polysilicon film 33 are flush with each other but V-shaped grooves are formed in the side wall 36 of the polysilicon film 33, resulting in a non-planar surface structure. If the quantity of etching is reduced so as to prevent the formation of the V-shaped grooves, then the SiO.sub.2 film 38 remains as a burr 39 or a film 40 completely covering the side wall of the resist 34 after the lift-off step as shown in FIG. 3F.
The above-described examples of the conventional lift-off technique employ such polymer as resist and PIQ, and they are subjected to a subsequent electrode wiring process in which heat treatment temperature is under 500.degree. C. at the highest. The resist and the PIQ are heat-resistant at temperatures below 200.degree. C. and 450.degree. C., respectively, but their compositions undergo a variation such as thermal decomposition or the like at higher temperatures. Accordingly, in the case where the resist or PIQ is not completely lifted off but partly remains unremoved in the abovesaid lift-off process, if the remainder does not act as a source of contamination in the subsequent manufacturing steps, the abovementioned material could be used. However, in the event that the subsequent manufacturing process involves high-temperature treatment for thermal oxidation, impurity thermal diffusion and so forth, contamination by the remainder of the lift-off material poses a serious problem.
Materials that have been studied as lift-off materials of higher heat resistance are aluminum and zinc oxide (ZnO), for instance. Aluminum has a melting point of 600.degree. C. but, at temperatures above 300.degree. or 400.degree. C., grain boundaries are grown in an aluminum film to increase irregularities of the film surface, making it impossible to form a miniature pattern. Accordingly, the temperature at which aluminum can be used as the lift-off material is 500.degree. C. at the highest. Furthermore, if aluminum remains unremoved after the lift-off process, it acts as an impurity source for silicon; therefore, aluminum cannot be used in the manufacturing process that subsequently involves a step of high temperature heat treatment at 800.degree. to 1200.degree. C. Since zinc oxide is resistant to heat above 500.degree. C. and soluble in a 1% phosphoric acid solution, it is used for lifting off a niobium compound which is a superconducting material. At 850.degree. C. and above, however, the zinc oxide reacts with SiO.sub.2 to form Zn.sub.2 SiO.sub.4, so that if the zinc oxide partly remains unremoved after the lift-off process, the subsequent high temperature heat treatment cannot be performed with an SiO.sub.2 film formed to underlie or overlie it.
As described above, according to the conventional liftoff method, the lift-off materials are all used in a low temperature process under 500.degree. C. at the highest. Accordingly, in the case where the lift-off material remains unremoved in the conventional lift-off process, contamination or reaction occurs in the subsequent high temperature heat treatment in the range of 800.degree. to 1200.degree. C., resulting in lowered yield of device fabriaction. For this reason, the prior art lift-off technique cannot be employed in the manufacturing process which involves high temperature heat treatment after the lift-off process.